The present invention relates to a method of and apparatus for nuclear quadrupole resonance testing a sample, and is applicable in one aspect to imaging a sample based on its nuclear quadrupole resonance (NQR) response. The invention has particular application to the detection of the presence of a given substance in a sample, and more particularly to the determination of the position and/or amount of material.
Nuclear Magnetic Resonance (NMR) techniques are now widely used for imaging, particularly medical imaging, e.g. using proton resonance. However, NMR investigations require a strong and highly homogeneous static magnetic field to operate, and this requires bulky and expensive equipment. In addition, due to the strong magnetic field, the method cannot be used in the presence of ferrous objects.
Nuclear quadrupole resonance (NQR) responses can be detected without requiring the presence of a strong static magnetic field, and so interest in using the NQR response of a body to probe its structure has recently developed. However, because NQR is a different phenomenon to NMR, existing NMR techniques cannot be directly applied to NQR investigations.
NQR testing has been increasingly widely used for detecting the presence or disposition of specific substances. The phenomenon depends on transitions between energy levels of quadrupolar nuclei, which have a spin quantum number I greater than or equal to 1, of which 14N is an example (I=1). 14N nuclei are present in a wide range of substances, including animal tissue, bone, food stuffs, explosives and drugs. The basic techniques of NQR testing are well-known and are discussed in numerous references and journals, so will only be mentioned briefly herein.
In conventional Nuclear Quadrupole Resonance testing a sample is placed within or near to a radio-frequency (r.f.) coil and is irradiated with pulses or sequences of pulses of electro-magnetic radiation having a frequency which is at or very close to a resonance frequency of the quadrupolar nuclei in a substance which is to be detected. If the substance is present, the irradiant energy will generate an oscillating magnetization which can induce voltage signals in a coil surrounding the sample at the resonance frequency or frequencies and which can hence be detected as a free induction decay (f.i.d.) during a decay period after each pulse or as an echo after two or more pulses. These signals decay at a rate which depends on the time constants T2* for the f.i.d., T2 and T2e for the echo amplitude as a function of pulse separation, and T1 for the recovery of the original signal after the conclusion of the pulse or pulse sequence.
The present invention, in one aspect, is particularly concerned with probing a sample to obtain information dependent on the position or distribution of resonant nuclei within a sample. This may be used to produce an image of the sample.
It is known that the NQR response of nuclei in a crystal is dependent on the environment of the nuclei, and also on factors such as the strength of the exciting field. If the exciting radio-frequency (r.f.) field strength varies throughout the sample, then the resonance response will also be dependent on position within the sample, and this can in principle be used to give an indication of the position of resonant nuclei within a sample.
A method for obtaining positional information using NQR, employing an r.f. field gradient, and not requiring a static magnetic field, has been proposed by Rommel, Kimmich et al. (Journal of Magnetic Resonance 91, 630-636 (1991) and also U.S. Pat. No. 5,229,722). Those disclosures (see page 631, line 25 of the paper and column 6, lines 46-50 of the patent) teach that NMR techniques such as phase-encoding (in which both the phase and the amplitude of the r.f. signal received from the sample are used to obtain information about the sample) cannot be applied to NQR imaging, and that only amplitude encoding is possible with NQR imaging. In other words, it is stated that it is only possible to extract a single parameter (signal amplitude) from an NQR imaging experiment which uses an r.f. field gradient in the absence of a static magnetic field. This is stated to be consistent with the theory that the transverse magnetisation associated with an NQR response oscillates, in contrast to the precession about the applied magnetic field observed in an NMR experiment.
Our earlier United Kingdom Patent Number GB-2,257,525 discloses a method of imaging using NQR in which a field gradient is imposed upon a sample. Reference should be made to that disclosure for useful background information and further discussion of the art of imaging using NQR which is not repeated here. In that patent, surprisingly advantageous results were obtained by subjecting a sample to a field having a particular positional dependence. Although that method can enable a satisfactory image to be obtained, there is still some room for improvement.
The present invention seeks to provide a method and apparatus for probing a sample by detecting its NQR response which alleviates some or all of the drawbacks of previous methods. Preferred arrangements disclose a probing technique in which positional information may be obtained even in the absence of a controlled static magnetic field.
The invention is applicable to detection of quadrupolar nuclei (Ixe2x89xa71) and is particularly applicable to nuclei such as 14N (I=1) in which advantageous results can readily be obtained in the absence of a static magnetic field, but may be used for detecting other quadrupolar nuclei, for example I=3/2, I=5/2 etc. The invention is particularly applicable to polycrystalline samples, or samples containing one or more polycrystalline clusters of quadrupolar nuclei.
In developing the invention, it has been appreciated that there are many NQR applications in addition to imaging in which it would be desirable to obtain more information than signal amplitude, but this has hitherto not been possible from a single measurement.
Surprisingly, the inventors have found that two independently varying components (e.g. phase and amplitude dependent components) can be extracted from a received NQR response signal if the excitation is selected appropriately. A preferred method of achieving this is to use two excitation pulses of selected phase. This can lead to a more reliable classification of the object under test.
The prior art has not reported detection of two independently resolvable components resulting from NQR interactions. Indeed, theory predicts only a single component is to be found, and Rommel et al. states that phase encoding is not possible in NQR experiments.
The phase and amplitude dependent components may actually be phase and amplitude, but it is to be understood that references herein to phase and amplitude dependent components are intended to include components derived from or related to the phase and amplitude of the response signal without necessarily being directly representative thereof. In particular, the signal may be resolved into two components, both of which vary as functions of both phase and amplitude. For example, in a preferred arrangement, the received signal is (initially) resolved into two components having a quadrature relationship. Phase-related information may be obtained by combining the two components in a first manner (e.g. comprising determining a ratio of the components) and amplitude-related information may be obtained by combining the components in a second manner (e.g. comprising summing a function of the components).
The extra information obtainable by the provision of both phase and amplitude information in an NQR experiment may be useful in a number of ways, as will be understood by one skilled in the art based on the discussion below.
In an imaging experiment, the provision of both phase and amplitude information can provide better classification of the sample than the amplitude encoding alone technique of Rommel et al. where the received signal amplitude is dependent on both the position (as intended in the experiment) and also on the amount of resonant material present. This can alleviate one problem of Rommel""s technique that unless the amount of resonant material is known, it may not be possible to determine its position accurately, and vice versa.
The phase and amplitude information may be used together to improve signal to noise ratio in any of a number of experiments where only amplitude information was previously available.
Thus, based on the results of the above surprising finding, the invention provides, in one aspect, a method for nuclear quadrupole resonance testing a sample containing quadrupolar nuclei, comprising applying excitation to the sample, the excitation being selected to produce a response signal containing detectable phase and amplitude components resulting from nuclear quadrupole resonance interaction between said excitation and said quadrupolar nuclei, detecting the response signal, resolving the response signal into (independently variable) phase and amplitude dependent components and processing the response signal on the basis of both components.
In this specification, references to processing the response signal on the basis of both components are preferably intended to imply processing the response signal as if it were a function of two independent variables (phase and amplitude), and in particular may include resolving the response signal into two independent quantities (e.g. phase and amplitude). This is to be contrasted with processing (for example in which a phase sensitive detector may be employed to detect signals of a particular phase) in which phase information is not measured as an independently varying quantity.
Preferably a plurality of values of the response signal are sampled (preferably for different excitation conditions e.g. pulse lengths) and a plurality of values of a phase parameter (e.g. phase or the ratio of real and imaginary components) varying as a function of the phase of the response signal substantially independently of the response signal amplitude are determined. Determination of variation of a phase parameter (preferably in addition to determination of an amplitude parameter) for several values of the response signal may enable useful information, e.g. positional information or information useful in noise reduction to be gained from the response signal.
According to a preferred arrangement, the excitation comprises first and second pulses differing in phase by a predetermined angle. This can provide a convenient method of exciting the desired response in a predictable manner.
The angle is preferably about 90xc2x0 as this may enable two substantially independent components to be resolved.
The two pulses are preferably transmitted from the same coil (or coils); this may provide convenience and ease of establishing phase correlation between the pulses.
The pulses may be separated by a time period, which is preferably relatively short, e.g. substantially shorter than the f.i.d. (Free-induction decay) time, T2*, and preferably zero or as close to zero as possible; that is, the pulses are preferably contiguous. A composite pulse is preferred, the first and second pulses being substantially contiguous but differing in phase; this may shorten overall measurement times, and may improve response signal amplitudes.
In a preferred arrangement, the excitation comprises two pulses of substantially equal duration, but different phase. Use of pulses of equal duration may simplify calculation of position (where position is determined) of the responsive material or other processing of the data. However, if the durations differ, and preferably if the response signal is determined for a plurality of different relative durations, this may be useful in obtaining a more precise determination of the position of a responsive substance in a sample.
The pulses are preferably of substantially equal amplitude; this simplifies the equipment needed and may simplify processing of the data.
It is particularly advantageous if the second pulse is arranged (at least partially) to lock the magnetisation (of the quadrupolar nuclei) generated by the first pulse. Such a sequence may be termed a xe2x80x9cspin lockingxe2x80x9d sequence, with the magnetization being locked for a time longer than would be achievable with the equivalent single pulse. Locking can be achieved by keeping the B1 field of the second pulse parallel to the magnetization produced by the first pulse. This may enable a stronger and longer lasting signal attributable to the first pulse to be detected.
Preferably also, the excitation includes a third pulse selected to lock (at least partially) the net magnetization produced by the first two pulses, and preferably being of phase intermediate that of the first and second pulses. This can further assist in locking the magnetization, and may result in a higher signal to noise ratio or better interference suppression. It may also be useful in selecting components of a particular phase, and this may be useful in selecting signals emanating from a particular region of the sample. This may also be provided as a further independent aspect, in a method of detecting NQR response signals emanating from quadrupolar nuclei in a given region of a sample, the method comprising exciting the sample to produce a response signal from the quadrupolar nuclei having a phase varying as a function of the position of the nuclei and identifying signals of a given phase, wherein said identifying preferably comprises applying a pulse arranged to lock response signals of said given phase.
Although reference is made above to a single series of two or three pulses, it will be understood that several series of excitation pulses may be used, and signals detected after excitation with some or all of said series. For example, a series of pulses may be used to lock spins. This may be useful in reducing interference or spurious signals due to other objects (e.g. metallic objects, particularly nickel plated objects) within a sample.
It will be understood that references in the present specification to phases differing or being equal is equivalent in certain circumstances to references to frequency differing or being equal, in that a change in phase implies an at least momentary change in frequency and vice versa.
In one practical arrangement, a phase-sensitive detector may be employed to detect the phase and amplitude dependent components as two components having a pre-determined phase relationship, most preferably a quadrature phase relationship. This may provide a convenient way of detecting two components in the signal. With this arrangement, the first and second components may correspond to the components along the x and y axis of the rotating frame, and in what follows these will be referred to as the real and imaginary components of the received signal.
Preferably a parameter varying as a function of phase is obtained from a ratio of the two components. This may enable simple but effective determination of a phase parameter.
Most preferably, at least the field strength of the excitation varies throughout at least a portion of the sample according to a given pattern. This is similar to the case with surface coils, well-known in magnetic resonance imaging, and provides a readily implementable way of providing a response signal which encodes or conveys information concerning the distribution or position of responsive nuclei in the sample to be obtained. In a preferred arrangement for achieving this, the excitation pulses are transmitted to the sample from a coil which produces a non-uniform r.f. field in the vicinity of the sample. The r.f. field amplitude preferably varies with position in a known manner.
In a preferred arrangement, the field strength and duration are selected to produce a variation of flip angles within a range of 0 to 2xcfx80 (360 degrees) throughout a region of interest of the sample, and preferably throughout the sample. The minimum usable flip angle will depend on noise and other considerations, but in some cases may be of the order of a few degrees. Preferably the maximum flip angle in the sample (or at least the region of interest) does not exceed 2xcfx80. Keeping all flip angles below 2xcfx80 may allow the measured phase to be a single-valued function of position, which may simplify determination of position.
It will be appreciated by those skilled in the art that the flip angle for a given pulse duration and amplitude is dependent on I, the nuclear spin quantum number as well as on the gyromagnetic ratio; the spin quantum number I affects the order of the Bessel function which governs the variation of effective flip angle with pulse duration and amplitude. In this specification, a flip angle of 2xcfx80 is intended to refer to a return of the magnetisation vector M into parallellism with its original orientation (which equates to a particular product of pulse amplitude and duration), and other flip angles are to be construed accordingly in proportion.
Preferably, position information representative of the position of said nuclei is obtained based on at least the phase of the response signal. Using phase (rather than amplitude alone) to determine position may enable positional determination to be substantially independent of the amount of responsive nuclei present. This may facilitate accurate determination with fewer measurements, for example in some cases a single measurement may suffice.
Preferably, quantity information representative of the amount of said nuclei is obtained based on at least the amplitude of the response signal, or based on the combined amplitude of two (preferably orthogonal) components into which the signal is resolved. This can be used in adjusting the results to take into account the amount of responsive nuclei, and, if combined with positional information, can allow a distribution of nuclei in the sample to be calculated.
Preferably, the received components are analysed to obtain profile information representative of the distribution of said nuclei in said sample. This may be useful in locating NQR responsive substances within a body, and may be developed to provide an image of the interior of the body.
The received components may be analysed to obtain profile information representative of variation of an environmental parameter, preferably temperature or pressure, which affects said NQR response in said sample. This may be useful in thermal or stress analysis.
Whilst both depth and position information can be obtained from the response to a single excitation (a composite pulse or pair of pulses) for a simple sample as discussed above, in an advantageous development, the excitation is applied repeatedly to the sample, and the analysis is repeated (preferably with at least one factor affecting the response varying as the excitation is repeated) to obtain a plurality of sets of said profile information. This may enable more accurate analysis of the sample, and preferably at least one further set of profile information of higher resolution and/or higher signal-to-noise ratio is obtained from said plurality of sets.
The factor is preferably at least one of excitation pulse duration and excitation field strength. This may provide an easily implementable method of optimising the excitation or obtaining multiple measurements.
For example, one or more of the pulse length and B1 field may be varied in a number of steps over the range of flip angles selected, in which case the resolution will be determined by the number of steps in each experiment, the greater the number of steps, the greater the resolution.
The duration of the pulses may be varied, for example in a series of increments. This may be used to extend the measurable depth over which nuclei can be detected or to improve the resolution at which determination can be made or to resolve ambiguities resulting from analysis of data from a single measurement or to improve the signal to noise ratio.
The relative durations of the first and second pulses may be varied, and preferably the total duration of the two pulses is kept substantially constant. For example, a series of measurements may be made, ranging from a relatively short first pulse and long second pulse through substantially equal durations to a relatively long first pulse and short second pulse. This may be useful in distinguishing signals from a particular location more accurately.
The amplitude (field strength) of the exciting pulse may be varied. This is preferably in a series of discrete increments, but may be substantially continuous or quasi-continuous in certain cases. This may enable accurate resolution of position, or may enable uncertainties or degeneracies in the distribution to be resolved, and may be particularly useful in reducing noise which may be present when the sample is distant from the transmitter coil(s).
The method may include obtaining a plurality of sets of profile information, corresponding to profiles at different positions or in different directions. This may be used in further characterisation of the sample, or determination of crystal orientation, and may be useful in imaging.
The excitation may be applied from two or more directions, preferably substantially orthogonal, and said profile information obtained for each direction. For example, the sample may be probed from different (e.g. 3 orthogonal) directions; this may be useful in obtaining a composite 3-dimensional image.
The sample may be physically moved with respect to the coil (i.e. by moving either or both of the sample and the coil). This is simple to implement, and has the advantage that a direct correlation between physical position and observed readings can readily be obtained. It may be useful, particularly in combination with other methods discussed below, for scanning in one direction for example to assemble a 3D image from a series of 2D slices, where an article is already moving. It may be useful, for instance, for imaging packages on a conveyor belt.
Thus, the profile information may be used for forming an image of the sample, the method further comprising constructing an image of the sample from at least one set of profile information.
In addition to imaging of the distribution of material in an object, the invention may also be applied to characterisation of temperature profiles within a sample. Alternatively, the method may be used for characterisation of other parameters which affect the resonance response of a sample, for example pressurexe2x80x94this may be used for example to produce a stress profile of a sample. Other applications will be apparent to those skilled in the art.
Thus, in an important second aspect, the invention provides a method of forming an image of a sample containing quadrupolar nuclei, the method comprising applying excitation to the sample, the excitation having a field strength preferably varying according to a given function of position and being selected to produce a detectable response signal resulting from NQR interaction between the excitation and the quadrupolar nuclei, the response signal being resolvable into phase-dependent and amplitude-dependent components, resolving the response signal into two received components representative of said phase-dependent and amplitude-dependent components and, based on both received components, producing an image representative of the distribution and/or environment of said nuclei in the sample.
Preferably the excitation is repeated a plurality of times (preferably at least 10, 20, 50, 100, 200, 500, 1000 or more times) and at least one of excitation pulse amplitude and excitation pulse duration is varied as the excitation is repeated. This may yield a set of received components which may be processed to produce an image. The step of producing an image may include transforming the data (from a plurality of repetitions), for example according to a Hankel transform or a Fourier transform, or may include correlating the data to a distribution pattern which would be expected to produce similar data, for example by a Maximum Entropy Method.
Preferably the position of responsive nuclei is determined based on a phase parameter which varies as a function of phase of the received components and is determined either from the phase of the received signal or from a combination of two received components which vary with both phase and amplitude of the received signal, for example from a ratio of two quadrature components.
A visual output may be produced of the image.
The second aspect may use any of the preferred or optional features of the previous aspect, and may include the previous aspect.
In addition to the imaging and profiling facilitated by use of phase, as briefly mentioned above, the phase information provided by the method of the first aspect may be used to suppress noise (this may still be applied where the phase information is additionally used in imaging). Thus, a preferred method includes obtaining a phase parameter (which varies as a function of phase of the detected signal) from the resolved components, and processing both resolved components using the phase parameter to produce an output having a signal-to-noise ratio greater than that of the response signal amplitude.
This important feature may be provided as a third aspect in a method of probing a sample to detect quadrupolar nuclei therein, the method comprising applying excitation to the sample, the excitation being selected to stimulate a response signal having detectable phase and amplitude components resulting from NQR interactions with the quadrupolar nuclei, detecting the response signal and resolving the detected signal into phase-dependent and amplitude-dependent components, obtaining a phase parameter from the resolved components and processing both resolved components using the phase parameter to produce an output having a signal-to-noise ratio greater than that of the response signal amplitude.
Preferably, the processing includes identifying mutually inconsistent values of the resolved components as representative of spurious signals. This may provide an efficient way of filtering out spurious signals. The phase parameter obtained with the invention is unique to NQR response signals, as it depends on the polycrystalline nature of the sample and the known dependence of the transition probability on the orientation of B1 in the electric field gradient frame of reference; piezo-electric responses or acoustic ringing will not exhibit the same phase relationships.
The processing may include applying a first excitation to produce a first received signal in which a desired signal has a first phase dependence, and applying a second excitation to produce a second received signal in which the desired signal has a second phase dependence, and detecting the desired signal on the basis of the first and second received signals and corresponding measured phase dependence thereof. Thus, the desired signals may be found by looking for a particular phase xe2x80x9csignaturexe2x80x9d.
Preferably also, the true quadrupole resonance signal is distinguished from any spurious signal in dependence on its (time) gradient, curvature or shape, perhaps in dependence upon whether the true and spurious signals have gradients of opposite sign.
The preferable and optional features discussed above in relation to other aspects may apply to this aspect, as will be well-understood by one skilled in the art.
The excitation may be varied to enable reliable imaging for a variety of environmental parameters (e.g. temperature), as discussed in our earlier patent application published as GB-A-2,284,898.
The above aspects may provide reliable methods for obtaining positional information.
The accuracy of the positional information obtained may be enhanced by determining the distances of a cluster of responsive (quadrupolar) nuclei from two or more reference points and calculating positional information based on the respective distances.
This can be provided independently, and according to a fourth aspect, the invention provides a method of determining the position of quadrupolar nuclei in a sample comprising applying excitation to the sample to produce a detectable NQR response, detecting a first response signal from said nuclei and determining a first distance of the nuclei from a first reference point; detecting a second response signal to determine at least a second distance of the nuclei from at least a second reference point; and determining positional information of said nuclei on the basis of said distances from said reference points. The first and second response signals may be detected by separate receiver coils at positions corresponding to the reference points.
The positional information may actually be the position of the nuclei in a particular reference frame, but the term xe2x80x9cpositional informationxe2x80x9d is intended to include any position-related parameter; for example velocity or acceleration may be determined by such a method.
Preferably, the positional information is determined by triangulation, and in a preferred arrangement a third distance from a third reference point is determined.
A plurality of coils may be used for transmission and/or reception of signals, each coil preferably being associated with a corresponding reference point; preferably a plurality of receiver coils are used to detect the response signal produced after excitation from a transmitter coil arrangement.
The detection of the distances from each reference point may be sequential or simultaneous.
The fourth aspect may be used independently, but preferably is combined with one of the earlier aspects; this can provide a more accurate indication of position. Most preferably, the excitation is arranged so that the phase of the response signal varies with the position of responsive (quadrupolar) nuclei with respect to a transmitter coil, and preferably both the phase and amplitude of the detected signal from each of a plurality of receiver coils is used to determine positional information.
The above methods may be applied to detection of a single substance at a single resonance frequency. It is also possible, and may be highly desirable in certain applications to repeat the measurements for a variety of different frequencies, corresponding to resonant frequencies of various substances of interest. For example, a sample may be scanned at frequencies corresponding to the resonant frequencies of one or more known explosives or components of explosives and/or at frequencies corresponding to one or more known narcotics or narcotic components.
Alternatively, the frequencies may correspond to resonant frequencies of biological substances of interest in a patient. The results of each scan may be combined to produce a better characterisation of the sample under test, for example by overlaying images obtained from each scan. This may produce a composite image (which may be displayed as a colour-coded image) identifying particular regions of interest within an article. In addition the results of one or more such scans may be combined with or compared to characterisation, such as images, obtained by other methods, for example X-Ray imaging.
In addition to the substances discussed above, the above NQR testing or imaging aspects of the invention may be applied to detection of proteins, hormones and other constituents of a human or animal body, for instance for medical imaging. Of particular interest in this respect is detection of 127I (I=5/2), which is present in thyroxin.
Surprisingly, it has been found that although nitrogen is readily detectable in compounds such as explosives using NQR testing, and is present in most biological compounds including proteins, detecting the NQR response of iodine in a biological compound or complex containing iodine (and nitrogen) may give advantageous results.
Based on this surprising finding, in a fifth aspect, the invention provides a method for NQR testing a biological specimen containing a particular substance containing iodine nuclei and preferably other quadrupolar nuclei (most preferably nitrogen), comprising applying excitation to the specimen, the excitation being arranged to produce an NQR response from the iodine nuclei, and detecting the response signal from said iodine nuclei (if present). Although detection of nitrogen in substances such as explosives has been found to work well, it has hitherto been troublesome to detect biological substances such as proteins from their NQR response.
This may be used in conjunction with the other aspects and preferred features described herein, and may in particular be used in conjunction with the imaging methods described.
Particularly useful results are obtained if the substance is thyroxin or a thyroxin derivative, precursor or analogue, and preferably wherein the specimen includes a mammalian thyroid gland. As well as 127I, other quadrupolar nuclei such as 35Cl (I=3/2) may be detected in a similar way. This may be particularly useful in xe2x80x9ctaggingxe2x80x9d experiments where it is required to follow the rate of uptake or loss of a given tagged species, for example in the thyroid gland or other organs such as the kidneys or liver.
The invention also provides apparatus arranged to perform all methods disclosed herein.
In a sixth aspect, the invention provides apparatus for detecting an NQR response in a sample containing quadrupolar nuclei, the apparatus comprising excitation means arranged to generate an excitation signal capable of exciting an NQR response having detectable phase and amplitude components; transmission means arranged to transmit the excitation signal to the sample; detection means arranged to detect a response signal generated by the sample to produce a detected signal; resolving means arranged to resolve the detected signal into first and second components; signal processing means connected to the resolving means to receive both components for processing the response signal based on both phase-dependent and amplitude-dependent components thereof; and control means for controlling operation of the apparatus.
The excitation means may be arranged to generate at least two pulses differing in phase by a predetermined angle, preferably 90 degrees, or other advantageous excitation waveforms discussed above in relation to the method aspects.
The transmission means may be arranged to generate a radio-frequency field having a field strength varying according to a given pattern throughout at least a portion of the sample; this may enable positional information to be detected from the response signal. The control means may be arranged to cause the transmission means to generate a plurality of said given patterns. This may enable several measurements to be made under different conditions.
The transmission means may comprise at least first and second coils (for example of different sizes and/or at different positions or orientations) for producing respectively, on excitation with a radio-frequency electrical signal, at least first and second radio-frequency fields varying in strength as different functions of position in the vicinity of the sample, wherein adjustment of the relative amplitudes of electrical signal supplied to each coil alters the pattern of the net radio-frequency field. These may include a coil (e.g. a coil arrangement such as a Helmholtz pair) for generating a field having a substantially constant field strength in the vicinity of the sample. Such arrangements may facilitate application of a desired field pattern to a sample.
The apparatus preferably includes means to store or to calculate the or each given pattern to provide an estimate of transmitted radio-frequency field strength at a plurality of positions, and having weighting means for determining an adjusted value of received signal strength based on the received signal strength and the estimated field strength at a position in the sample corresponding to the source of the received signal. This may enable a more accurate determination of the amount of responsive nuclei in a sample from the received signal strength.
Preferably, the resolving means is arranged to resolve the received signal into components having a quadrature relationship, for example by employing a quadrature detector. This provides a convenient arrangement for producing two components from which a phase parameter can be determined.
The apparatus may include means for causing a variation in at least one environmental parameter which affects said NQR interaction throughout at least a portion of the sample. This may be used for further encoding of positional information.
Preferably, the signal processing means is arranged to sample the detected signal for a predetermined time, and to store two components which together contain both phase and amplitude information. This may facilitate determination of a phase parameter.
Each feature of each method aspect of the invention can be applied to the apparatus aspect as appropriate.
The phase information provided by the invention can be used in a number of ways including those discussed in more detail below. In another aspect, the invention provides use of the phase of an NQR response signal from quadrupolar nuclei in the determination of the position of the nuclei, or in imaging of a sample containing the nuclei, or in reduction of noise in the NQR response signal.